I recommend checking the formula references and the user guide for the gretl application because it requires extensive knowledge to be able to work with it. From what I've see on the official website, the user guide and formula reference contains everything about the software and the way it works or how you can work with it.
Access the following link for the website and this one for user guide and formula reference.
Note: You will also need a PDF reader to be able to open the user guide files.